Equilibrium Dynamics in the Thallium(III)−Cyanide System in Aqueous Solution

Abstract

Ligand exchange reactions of thallium(III) cyano complexes, Tl(CN)(n)()(3)(-)(n)(), have been systematically studied in aqueous solution containing 4 M ionic medium {[ClO(4)(-)](tot) = 4 M, [Na(+)](tot) = 1 M, [Li(+)](tot) + [H(+)](tot) = 3 M}, at 25 degrees C, using (205)Tl and (13)C NMR one-dimensional inversion transfer techniques. Rate constants for all dominating exchange pathways were determined and compared to the previously studied thallium(III) halide complexes. Also in the case of cyanide ligands the ligand exchange is dominated by the rare type of reactions occurring via a direct encounter of two complexes (self-exchange reactions), e.g. Tl(CN)(3) + Tl(CN)(2)(+) right harpoon over left harpoon Tl(CN)(2)(+) + Tl(CN)(3) (k(32), k(23)) or Tl(CN)(2)(+) + Tl(CN)(4)(-) right harpoon over left harpoon 2Tl(CN)(3) (k(24), k(33)). The determined cyanide exchange rate constants between complexes Tl(CN)(m)()(3)(-)(m)() and Tl(CN)(n)()(3)(-)(n)(), for the rate-determining step have all similar values, about 100-1000 s(-)(1), that are 5 orders of magnitude smaller than for the corresponding halide exchange processes. This indicates the presence of a common rate-determining step for the self-exchange reactions of the cyanide ligand, proposed to be the breaking of the thermodynamically very stable Tl-CN bond. This is in contrast to the Tl(III)-halide systems, where the breaking of the Tl-OH(2) bond was proposed to determine the reaction rate. The second type of cyanide exchange, namely anation, has been found only in two cases: Tl(CN)(2)(+) + CN(-) right harpoon over left harpoon Tl(CN)(3) (k'(23), k'(32)); and Tl(CN)(3) + CN(-) right harpoon over left harpoon Tl(CN)(4)(-) (k'(34), k'(43)). These reactions are very fast, k'(mn)() approximately 10(9) M(-)(1) s(-)(1), and are proposed to proceed through an associative interchange mechanism, where the rate-determining step is a water dissociation mediated by the incoming ligand, i.e. similarly as for the corresponding halide complexes. The third type of cyanide exchange reactions was possible to study due to the presence of an NMR-active nucleus ((13)C) in the ligand. Only the following ligand substitution reaction was observed: Tl(CN)(2)(+) + HCN right harpoon over left harpoon Tl(CN)(2)(+) + HCN The reason for the dominant role of the self-exchange reactions is the very low concentration of free CN(-) and the inertness of the HCN species in the ligand exchange reactions. The obtained dynamic information is discussed and compared to the corresponding data for the thallium(III) halide complexes.